Currently, fossil fuels are used as a main energy source. However, fossil fuels are finite. Furthermore, fossil fuels have a problem that carbon dioxide generated when it is used increases greenhouse effect. Therefore, development of energy source in place of fossil fuels is desired. One of new energy sources includes a fuel cell.
Compared to primary batteries and secondary batteries, a fuel cell is a power generator semipermanently usable by continuously supplying hydrogen and oxygen as fuel. A fuel cell has attracted also because the used fuel can be reused. Among them, polymer electrolyte fuel cell (PEFC) operates at low temperatures, and reduction in size and weight is possible since its electrolyte is thin-film like. Thus, PEFC is expected to be applied to home electric appliances, mobile devices, automobile battery, and the like. PEFC has a structure in which an electrolyte film is sandwiched between two electrodes, cathode (positive electrode) and anode (negative electrode). In PEFC, fuels, such as oxygen to a positive electrode and hydrogen to a negative electrode, are supplied, and an electric energy can be obtained from a chemical reaction caused in the electrode.
The cathode of the fuel cell carries an electrode catalyst, and catalyzes a reaction to reduce oxygen to water. The reaction rate of oxygen reduction reaction on the cathode side is relatively low, thus a catalyst for efficiently operating the reaction is necessary. As the electrode material, carbon-based electrode materials and the like are known, and a platinum-containing catalyst is currently most effective as a carbon-based electrode catalyst for efficiently operating a fuel cell. However, since platinum is a noble metal, a problem on costs is pointed out. Therefore, creation of a novel catalyst not using platinum has been expected.
Incidentally, the important thing in the creation of the carbon-based electrode catalyst is to create a carbon material having high conductivity, wide surface area, and good dispersibility, and metal is finely dispersed in the material. As one of the base material of such carbon material, phthalocyanine is known (for example, refer to Patent Literature 1). The carbon material described in this literature is obtained by calcining hyperbranched metal phthalocyanine including a specific repeating unit in an inert gas atmosphere. The metal ion constituting phthalocyanine core of this repeating unit is selected from the group consisting of Fe2+, Co2+ and Ni2+, thus is characterized in that it is not necessary to use expensive noble metal such as platinum.
Phthalocyanine is known to include many coordinating elements for fixing metal. Since phthalocyanine has a giant cyclic structure in which the whole molecule forms a conjugated double bond system, the structure and bonding thereof are extremely stable, and phthalocyanine coordinates with a metal ion such as transition metals at its center, and phthalocyanine forms a stable metal phthalocyanine complex. The advantages of using metal phthalocyanine as an electrode material include that it can stably fix metal, namely, it can be suggested that metal arrangement can be controlled at nano level. Furthermore, the advantages of using metal phthalocyanine as a precursor of a metal carrying carbon material include that the carbon content is high. Namely, when phthalocyanine is calcined to forma carbonized material, the carbon content of the electrode can be enhanced.
However, in the catalyst material described in Patent Literature 1, a constituent unit derived from phenols is required other than metal phthalocyanine, as described in the formula (I) of the paragraph 0023. Namely, hyperbranched metal phthalocyanine of Patent Literature 1 has a bonding in which the constituent unit derived from phenols is interposed (—O—Ar—O—) or a bonding in which it is not interposed (—O—) between adjacent metal phthalocyanines, thus lacks the regularity of metal arrangement. Also, it is preferred to have a simpler constituent unit, from the viewpoint of industrialization. On this point, there was a room for improvement on the compound described in Patent Literature 1.
In addition, as a more preferred method for search of a cooperative effect of a different metal, there was a room for improvement on the invention described in Patent Literature 1. More specifically, the hyperbranched metal phthalocyanine described in Patent Literature 1 contains only one type of metal phthalocyanine, thus cannot be used as a precursor compound for searching a cooperative effect of two or more types of metals.
Thus, the inventors have so far prepared a metal phthalocyanine derivative by condensation of a metal aminophthalocyanine compound and a metal carboxyphthalocyanine compound (for example, refer to Non Patent Literature 1). In this literature, the inventors have reported that a carbon material obtained by calcining the metal phthalocyanine derivative under specific conditions has a catalyst activity of oxygen reduction. As described above, the metal phthalocyanine derivative described in Non-Patent Literature 1 includes simple constituent unit in which metal is regularly arranged, and can be preferably used also for search of a cooperative effect of a different metal as necessary.